The invention relates to a method for dissolving ores which contain base metals that would, if contacted with a hydrohalic acid evolve hydrogen, and noble metals in either metallic or alloyed form, or both.
It has long been known to treat ores which contain base metals as well as noble metals by first dissolving the base metals with hydrochloric acid, and then dissolving the noble metals in a solution of hydrochloric acid and chlorate. A disadvantage of this method is that hydrogen is released, which has a high potential for deflagration and endangerment.
Furthermore, it is known to dissolve such ores with aqua regia. In this case too large amounts of hydrogen are released, with the disadvantages referred to above, and nitrogen oxides are also formed, which is a concern with respect to environmental contamination, and energy-intensive concentrations of hydrochloric acid are required for a subsequent operation to remove nitrates.
On this account the problem arises of at least partially remedying the above-mentioned disadvantages by means of a novel process. The problem lies especially in developing a process for dissolving such ores which produces little exhaust contamination, no evolution of hydrogen, and eliminates the need for hydrochloric acid concentration.
This problem is solved according to the invention by a method wherein an ore containing base metals which, if contacted with hydrochloric acid, would result in the release of hydrogen, and also containing noble metals in metallic form, alloyed form or both, is placed in a solution of a non-acid oxidizer containing halide ions, and having a halide concentration of at least 1 mol/l, at a temperature of 70xc2x0 C. to 90xc2x0 C. In this case the oxidizer, which contains halogen, will, when brought into contact with hydrohalic acid form elemental halogen, such as chlorine gas for example. Then acid forming halide ions in aqueous solution is added, dissolving the metal components. The addition of acid is performed such that no hydrogen forms, and thus it is adapted to the rate of the reaction.
The solution that is formed is then adjusted by the addition of hydrohalic acid to a minimum H+ concentration of 1 mol per liter. In this manner the oxidizer breaks down with the release of elemental halogen, especially a halogen gas.
It is especially advantageous if chlorate, bromate or hypochlorite is used as the oxidizer, since, with these oxidizing agents a rapid dissolution of the metal compounds takes place and the added H+ ions are buffered away at a satisfactory rate.
Preferred oxidizers are sodium chlorate, potassium chlorate, sodium bromate, potassium bromate, sodium hypochlorite and potassium hypochlorite.
The ore is advantageously put into a 4.5 molar solution of the oxidizer, since the addition of the solution for one thing prevents any uncontrolled deflagration by having a high heat removal ability and for another thing the volume added thereto does not disproportionately increase size of the reaction mixture due to the high salt concentration.
In practice it has proven particularly advantageous if the oxidizer solution is maintained at a temperature of 80xc2x0 C., since at this temperature the speed of the reaction is high enough to capture the H+ ions (preventing the formation of hydrogen gas) and dissolve the metals; while on the other hand the solubility of the chlorine gas that forms is still sufficient to keep it in solution.
Also, solid sodium chloride is advantageously added to the oxidizing solution so as to provide a sufficient concentration of chloride ion throughout the entire process to minimize the danger of a hazardous formation of chlorine dioxide. The only oxidation product that forms, in addition to the actual metal ion compounds, is a chlorohalogen, especially chlorine gas.